This invention relates generally to magnetic deflection yokes as used in cathode ray tubes (CRTs) for displacing an electron beam therein and is particularly directed to the assembly and fabrication of CRT magnetic deflection yokes employing magnetic field formers.
An electromagnetic deflection yoke as used in a CRT for controlling the position of an electron beam on the CRT's faceplate generally includes two pairs of deflection coils. One pair of coils provides for vertical deflection of the electron beam, or beams as in the case of a color CRT, while the other pair of deflection coils provides horizontal electron beam deflection. The deflection yoke is generally disposed around the neck and funnel portions of the CRT and is securely mounted thereto so as to generate a precisely controlled magnetic field within the CRT which acts upon and displaces the electron beam therein in a predictable manner. The electron beam is deflected at a first, higher rate horizontally and at a second, lower rate vertically so as to trace a raster scanned pattern upon the CRT's faceplate.
In general, a pair of ferrite cores are symmetrically disposed around the electron beams and oriented generally transverse to the direction of travel of the electrons. A pair of vertical windings are wrapped around each of the ferrite cores and provide for varying the magnetic field to allow the electron beam to be deflected upward in a stepwise manner after each horizontal sweep followed by full vertical deflection of the electron beam after the CRT's faceplate has been completely traced to re-position the electron beam for initiating another raster scan trace of the CRT's faceplate. A pair of horizontal deflection coils are positioned symmetrically about the axis of the electron beams and disposed within the ferrite cores for horizontally deflecting the electron beam across the CRT's faceplate. Additional electromagnetic components such as color purity coils and magnetic field formers are also typically included in the deflection yoke for exercising improved control over the electron deflecting magnetic field for enhanced video image sharpness, color and definition.
In order to generate magnetic fields of sufficient intensity, the aforementioned deflection coils must carry large currents. These large currents, in turn, produce substantial heat within the deflection coils and immediately adjacent deflection yoke components. Excessive heat causes thermal distortion of deflection yoke components resulting in beam landing errors on the CRT's faceplate. The deflection yoke must therefore be assembled with components and materials capable of withstanding a high temperature environment while remaining in a rigid, fixed configuration capable of providing stable, predictable magnetic deflection fields.
The typical deflection yoke includes a plastic housing comprised of two half portions which are securely joined by snap-acting connectors. The ferrite core also is generally comprised of two half portions which are positioned about the outer portion of the yoke housing and are maintained in position thereon by means of metal clips. The current carrying magnetic deflection coils are either wound about respective ferrite core portions (vertical deflection coils) or are positioned in intimate contact with and mounted upon the yoke housing (horizontal deflection coils). A synthetic resin in combination with a contact adhesive may be used for maintaining the aforementioned vertical and horizontal windings on the ferrite core and yoke housing as disclosed in U.S. Pat. No. 4,494,097 to Lenders. The synthetic resin is used as a filler and possesses a coefficient of thermal conductivity to provide for the dissipation of thermal energy produced in the deflection coils. The contact adhesive maintains the deflection coils in intimate contact with the yoke structure upon which they are mounted.
Similar arrangements have been used to position the aforementioned magnetic field forming structures on the deflection yoke with only limited success. The metallic field formers are particularly sensitive to "hot spots" and it is therefore essential that the heat to which these elements are exposed be minimized and that the associated thermal energy be distributed uniformly within these field forming elements. Prior art approaches in this area have suffered from the presence of gaps within the field former mounting arrangement which not only reduce heat transmission from the field former, but also reduce the support for and stability of the field forming element as mounted upon the deflection yoke. With the field forming element not securely mounted upon the deflection yoke, it is subject to low frequency vibration such as arising from the 60 Hz line voltage provided to the CRT. Vibration of the field forming element appears as jitter on the CRT's faceplate which in combination with deformation of the field forming element arising from hot spots seriously degrades video image quality. Finally, since the field forming elements are fairly large, generally extending substantially around the entire circumference of the deflection yoke housing, a substantial amount of adhesive material is required for securing the field forming element to the yoke. The use of such materials increases the cost of CRT yoke manufacture.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing rigidifying means for use in a CRT deflection yoke comprised of a hot melt, foamed adhesive which is extruded in liquid form on the field forming elements of the deflection yoke when in position thereon so as to securely and stably mount the field forming element on the deflection yoke. The foamed adhesive provides improved thermal conductivity and heat dissipation and increased mounting strength for the field forming element. By foaming the hot melt adhesive, the required amount of adhesive material may be substantially reduced, e.g., by as much as 70%, resulting in a corresponding reduction in deflection yoke cost.